OK, you figured that this thread would draw my out to howl a bit....
For the past ten years (and unknown for how long yet to come) I've
been a telecom engineer so I'm probably as qualified as anybody to
spout off on this subject.
First of all, the 56kbps limit on dial-up is dictated by physical
limits of the "Voice Switch Network". This is a hard limit determined
by the time-slicing that is imposed on the signal. We call this "TDM"
or Time Division Multiplexing. You have to think of how the telephone
system works here.
>From the customer premise to the "central office" you have a twisted
copper pair of wires. The twist is to address RFI (Radio Frequency
Interference), induction and capacitive issues. Depending on the age,
location, distance, etc., the guage of the wire varies, but is usually
22 guage give or take two sizes and is solid core copper. Contrary to
popular belief of audiofiles everywhere, this pair of copper is able
to pass frequencies well beyond 200mHz. Frequency/distance tables are
known for each guage of copper and is easily calcuatable. For short
distances, it is not uncommon to be able to push up to 10gbps over
copper. Even in your home or office, you usually have 100mbps LANs
which are copper, so copper isn't really the issue.
So, this is called the "access loop" or "access circuit". This is the
physical plant required to bring the customer circuit over the outside
plant to the central office or access multiplexer. At this point the
AD/DA converters bandpass limit the audio signal and digitizes to a
maximum of 56kbps (or 64kbps in some parts of the world). This signal
is then passed through the voice-switched network as 1/24th of a T1.
There are 28 T1s in a DS3 and 48 DS3s in a OC-48... As dial-up
Internet is transported directly through this switched network, there
is an absolute physical top limit to the bandwidth attainable through
this technology.
DSL technology is substantially different. What happens with DSL as
provided by the telephone company is that we have TWO audio bandbass
filters on each end of the copper access loop. The spoken word is
filtered out of the data signal and the data signal is filtered out of
the spoken word. These are standard-fair low and high-frequency cut
filters not unlike audio-crossovers in your stereo system. Everything
from about 10khz on down is passed to the telephone handset, while
everything from 10khz on up goes to the dsl modem. This occurs on both
ends. When a filter goes bad you get a fuzzy noise in the handset or
broken data connection everytime the telephone rings. At the central
office (or whever the DSLAM-Digital Subscriber Line Access
Multiplexer, is located), the data signal is separated from the voice
signal. The voice-signal is passed to the voice switch network and the
data signal is then passed on to a separate data network.
DSL uses a "spread-spectrum" type of encoding which in practical
application means that the higher the frequency response of the copper
loop, the greater the possible data throughput. Now remember that this
is distance limited, not really anything more than that. If the
distance is short enough, you get as fast of speed as would be
possible with any other transport technology (fiber). Once you get
past about 1500 meters, copper starts to fall off quite dramatically,
and then again about 6000 meters most ADSL technologies kinda of hit
the limit unless impedence/capacitance balancers are installed every
few thousand meters along the line. If the line is fully balanced, we
can double and sometimes triple the distance. But even so, you wont
get 18mpbs of throughput at 10000 meters.
We use different types of inverse-multiplexing technologies to achieve
customer-loop bandwidths exceeding 100mbps over copper pair/s. In
fact, were currently testing one which will deliver up to 1gbps up to
3000 meters over copper pairs.
So, in reality, the main advantage of fiber over copper is the ability
to deliver faster speeds FARTHER. It's not that fiber is any faster
(although it can be when you do wave-division multiplexing (WDM).
What about cable-modems? Are they inherently faster? Nope. Copper,
but the main difference there is instead of using copper pairs, it
uses a coaxal "conduit" which allows radio signals to be transmitted
through the copper with minimal signal attenuation. Maximum bandwidth
of DSL and Cable are about the same.
Why, then is the internet faster or slower depending on your location?
It doesn't matter how fast your DSL connection is if the ISP has
narrow straws upstream of the Access Multiplexer. You might have
10mbps of download spead, but if the DSLAM you are connected to has
only 3mpbs (two T1s) of backhaul, you will only get a maximum of 3mpbs
and that's only possible if nobody else is using it. Remember, your
copper pair is exclusively yours, but once you get to the first
multiplexer, you enter a shared environment and then you succumb to
the curse of stat-muxing.
As a general rule, we can go 20:1 on stat-muxing for most backhauls
and up to 200:1 on the primary drain. Unfortunately, many ISPs will
oversubscribe far higher than 20:1 on the backhauls and when they are
reaching 40:1 the customer complaints get pretty severe. We usually
see customers changing ISPs at 25:1 in competitive areas.
FTTH (Fiber to the Home) is the promise to the future of 100mbps and
higher access speeds, but the fact is, the backhaul network at that
point will be the limiting factor and as ISPs are for-profit
operations, and backhaul is extremely expensive, $15 to $25 per month
month per subscriber JUST for the backhaul and Internet drains.
AG
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